A stereoscopic display gives the illusion of depth in the image by giving each eye a different perspective of a scene, as would happen in reality. The brain then fuses these perspectives together to form a 3D representation of the image in the brain. For example, this may be done by displaying one perspective with one polarisation, and the other perspective in a different polarisation. A viewer can then see stereoscopic depth by wearing glasses where each eye piece only allows the appropriate polarisation to pass.
An auto-stereoscopic display is a display that gives stereoscopic depth without the user needing to wear glasses. It does this by projecting a different image to each eye. These displays can be achieved by using parallax optic technology such as a parallax barrier or lenticular lenses.
These types of displays are well known in the literature. For instance, the design and operation of a parallax barrier for 3D is well described in a paper from the University of Tokushima Japan (Optimum parameters and viewing areas of stereoscopic full colour LED display using parallax barrier, Hirotsugu Yamamoto et al., IEICE trans electron, vol E83-c no 10 Oct. 2000).
In summary, FIGS. 1a and 1b of the accompanying drawings shows the basics of the parallax barrier operation and design in a cross sectional diagram of an auto-stereoscopic parallax barrier design. The images for the left and right eye are interlaced on alternate columns of pixels, as for previous designs. The slits in the parallax barrier allow the viewer to see only left image pixels from the position of their left eye and right image pixels from the right eye, just as for a dual view parallax barrier.
The viewer may look on axis at the display to see a stereoscopic view, but note that they may also see a stereoscopic view off axis as shown in the Figure. The on axis view is termed the primary viewing window and the off axis view is called the secondary viewing window.
The same 3D effect can be achieved by using lenticular lenses. Each lens is substantially equivalent to a slit on the parallax barrier. FIGS. 2a and 2b show a conventional 3D system using lenticular lenses. The lenses image the pixels to the viewer (who is typically 300 mm from the panel). As shown in the diagram, light from the left pixels is directed into the observer's left eye, and vice versa. To achieve this, the focal length is typically set such that it is about equal to the lens-pixel separation distance (so that the focal length of the lens is approximately at the plane of the pixels).
This design works very well and has been used for many years to create good stereoscopic displays.
In a less common design for a 3D display, the pixels have large spaces between them. This causes problems with the design of a 3D display. For example, large spaces between the pixels might occur if the pixels in an LCD display panel need to be rotated by 90 degrees as shown in FIG. 3.
The black mask region has the function of covering the electronics that exist in the display. In an active matrix LCD display each pixel has an associated thin film transistor (TFT) which is masked by the black mask. These TFTs are usually positioned above each pixel and this explains why there is a larger region of black mask above each pixel in FIG. 1a, than to the left and right sides of each pixel. With the rotated pixel design of FIG. 3a the TFTs are to the right of each pixel so that there is a substantial gap to the right of each pixel. Such a design might exist in a games console where the screen is used in a landscape orientation.
The effect of the gaps if used with a standard lenticular design is illustrated in FIGS. 3a and 3b. With a conventional design the lenticular lenses are focused on the pixels and so create an image of the pixels in the plane of the observer. The light from the left pixel is imaged to the left eye and vice versa, but in between the observers eyes there is a region of darkness caused by the image of the black mask. If the viewer moves slightly off axis the display will appear dark as they move into the dark region created by the black mask and lenses. When viewing a 3D image, this reduces the freedom that the viewer has to move from side to side. In addition a 3D display may be used as a 2D display by making the left and right images the same. However with the dark regions that exist at some angles, the usability of the 2D display is also reduced.
One known solution to this problem is to de-focus the lenticular lenses by making their focal power stronger or weaker. This has the effect of blurring the images (called ‘viewing windows’) that are formed at the observers' eyes, as illustrated in FIGS. 4a and 4b. In this case, as the observer moves off axis, the change in brightness from the panel is improved. As the left eye moves off axis the left eye image will fade to black rather than suddenly turning to black. In addition, as the left eye image fades, the brightness of the right eye image increases to compensate. In this way, the brightness level from the panel can remain more constant.
The disadvantage of this solution is that, in order to decrease the brightness variation to a low enough level, a region is created where both left and right images are visible to the left eye (an ‘image mix region’). Therefore the observer sees a double image which makes the image of poor quality in this region, and the usable viewing freedom of the display in 3D mode is reduced. The advantage is that, when the display is used in 2D mode, the brightness variations are reduced. With this design, it is not possible to create a sharp transition from the left eye image to the right eye image (the ideal case for a 3D image) and remove the brightness variation from the panel.
Another potential solution to this problem is touched on in the description of U.S. Pat. No. 7,070,278 (4 Jul. 2006). This patent suggests that the lenticular lens could be split into sections. This is shown in FIGS. 5a and 5b. Regions ‘1’ of the lenticular are slightly offset to the left of the display whilst regions ‘2’ are offset to the right. If the viewer happens to be looking at the display from a position slightly off axis to the left, then regions ‘1’ will show a good bright image to the viewer. If the viewer happens to be looking at the display from a position slightly off axis to the right, then regions ‘2’ will show a good bright image to the viewer. In this way, for a good range of head movement at least some of the display will show a good 3D image. The lenses do not need to be defocused to provide this extra head freedom so that the transition between left and right images can remain sharp with no region where a double image is visible.
The disadvantage of this system is that, when the display is viewed slightly off axis, only one of the lens regions will show an image. The other lens region will appear black. This will cause only half the brightness of the display to be seen, and worse still, only half the resolution of the display will be seen. It may also be that, when viewed from exactly on axis, regions 1 and 2 show an image so that the display is full brightness and full resolution. In this case, the resolution and brightness will vary as the user moves from side to side which could produce a noticeable and distracting image artefact.
GB 2406730 proposes a directional display in which a lenticular screen cooperates with a display device to provide multiple views. Each lenticule of the lenticular screen has elongate parallel portions with displaced centres of curvature. The lenticules are arranged such that their outer portions direct light from adjacent non-aligned pixels to the zeroth order lobe.
US 2007/0183033 proposes a backlight for use with a transmissive spatial light modulator. The backlight includes a lenticular screen having lenticules with portions of displaced centres of curvature.
U.S. Pat. No. 7,002,748 proposes a packaging sheet for providing a visual effect to attract a viewer to a package wrapped in the packaging sheet. The packaging sheet comprises a lenticular screen printed on its rear flat surface with spatially interleaved images. In one embodiment each lenticule has a middle portion of larger radius of curvature than the adjacent side portions, such that the middle portion provide a “see-through” effect in that it does not image the printed image on the rear surface of the screen but instead allows viewing of an object below the screen.
U.S. Pat. No. 6,369,949 proposes a generally similar arrangement to U.S. Pat. No. 7,002,748, in the form of a lenticular screen on whose rear flat surface spatially multiplexed images are printed.
U.S. Pat. No. 6,130,777 proposes a rear projection screen in the form of a lenticular screen laminated to a diffuser. The diffuser comprises minute features such as lenticules.